The invention relates generally to an apparatus and method of imaging one or more vasculatures using a magnetic resonance (MR) system and, more particularly, to acquiring data for more than one k-space during a cardiac cycle.
When a substance such as human tissue is subjected to a uniform magnetic field (polarizing field B0), the individual magnetic moments of the spins in the tissue attempt to align with this polarizing field, but precess about it in random order at their characteristic Larmor frequency. If the substance, or tissue, is subjected to a magnetic field (excitation field B1) which is in the x-y plane and which is near the Larmor frequency, the net aligned moment, or “longitudinal magnetization”, MZ, may be rotated, or “tipped”, into the x-y plane to produce a net transverse magnetic moment Mt. A signal is emitted by the excited spins after the excitation signal B1 is terminated and this signal may be received and processed to form an image.
When utilizing these signals to produce images, magnetic field gradients (Gx, Gy, and Gz) are employed. Typically, the region to be imaged is scanned by a sequence of measurement cycles in which these gradients vary according to the particular localization method being used. The resulting set of received NMR signals are digitized and processed to reconstruct the image using one of many well known reconstruction techniques.
Contrast-enhanced MR angiography (CEMRA) is an MR technique used to create MR images to aid in the assessment of diseases such as aortic aneurysms, aortic dissection and pulmonary embolism. In addition, CEMRA has also been utilized for imaging the left atrium and pulmonary veins for pre-operative planning of cardiac ablation procedures. With CEMRA, k-space acquisition corresponds in time and space with the arrival of the maximum, or near maximum, concentration of a contrast agent. As a result, veins and/or arteries appear with greater contrast in resulting images. Unfortunately, CEMRA techniques may suffer from motion artifacts, many of which cause blurring. For example, CEMRA images of cardiac and thoracic vasculatures may suffer from blurring and pulsatility artifacts due to cardiac motion.
To reduce such artifacts, electrocardiogram (ECG) gated acquisition is commonly used in conjunction with CEMRA. Often, however, ECG gated acquisition results in an elongation of scan time in order to obtain desired image spatial resolution. As such, image spatial resolution may suffer if scan times are reduced.
With conventional CEMRA, visualizing a lone vasculature component, such as the arterial component, often requires the setting of a variety of imaging parameters. These parameters, which often require accurate settings, include the timing of the contrast-agent arrival, optimization of the imaging speed, and the determination of the imaging acquisition k-space ordering scheme. If proper resolution of individual vasculature components such as arteries and veins is required, all of the above-mentioned parameters need to be optimized and set correctly in order to minimize the presence of undesired vasculature components in the resulting image. If such imaging parameters are not optimized, there may be an overlap of vasculature components, which may lead to an erroneous image interpretation.
Further, if examination of several vasculature components is desired, a scan must be repeated multiple times, and the imaging parameters of each component must be optimized for each scan. For example, separate scans, each with optimized parameters, would need to be performed for each of the arterial components and the venous components. Due to this need to optimize such parameters, multiple-vasculature-component protocols frequently lead to severe compromises. One compromise may include a reduction in the spatial resolution to shorten scan time of the vascular component and allow sufficient temporal speed for proper segmentation of vascular transit. Another compromise may include an extreme extension of the acquisition length due to the need for serial scans.
Scan times may also have imposed constraints. In many cases, the total scan time available is limited due to patient limitations such as breath-hold capacity or to the inherent rapid speed of contrast bolus passage through the vasculature. Due to a limited scan time, a conventional CEMRA acquisition of multiple vasculature components may not be possible with a single bolus acquisition. In these cases, separate contrast injections may be required to image each vascular bed. For example, to image the arteries and veins associated with the cardiac vasculature with high spatial resolution and appropriate contrast, conventional CEMRA acquisition often requires separate injections for each venous and arterial region.
Another possible approach for multiple vasculature acquisition is a time-resolved CEMRA technique that uses an alternated k-space scheme, such as TRICKS or TWIST techniques. With such a time-resolved CEMRA technique, the central and peripheral k-space views are sampled in an interleaved manner with over-sampling of the central k-space data. The temporal dynamic images are later reconstructed using subsets of the central and peripheral k-space data. However, the spatial resolution and temporal resolution of these scans are limited by the breath-hold time. Further, when ECG gating is required, whereby the data is only acquired in a specific temporal window within the cardiac cycle, the achievable spatial resolution is even further limited.
It would therefore be desirable to design a system and method capable of imaging one or more vasculature components while achieving a robust image contrast of the vasculature components.